The field of the present invention relates to the use of high impact, lightweight, high optical-quality polymeric material in polarized plastic parts such as eyewear.
Optical-quality eyewear requires good optical performance. In the selection of lens materials for use in optical-quality eyewear, the color, weight, and safety of the material is important, as well as good optical performance. Most often, however, the respective properties of different materials necessitate trade-offs among the desired lens characteristics. For instance, glass has very good optical properties, but it is heavy (a dense material) and only impact resistant if thick (resulting in an even heavier lens). Polymeric thermoset resins, such as CR-39®, are lighter in weight but are lacking in impact resistance. Polycarbonate, in contrast, is both lightweight and highly impact resistant. Polycarbonate also has a high refractive index. Thus, thin lenses can be made utilizing polycarbonate. However, polycarbonate exhibits more chromatic aberration than glass, typically resulting in unacceptable off-axis distortion.
In light of the foregoing, an alternate material with both good optics and high impact resistance is desirable. In addition, a lightweight material is desired for the wearer's comfort, convenience, and fashion consideration.
U.S. Pat. No. 5,962,617 (“'617 patent”) describes an initial formulation of a prototype material, which the inventors recognized may provide an improved combination of lens characteristics. This material comprises polyurethane pre-polymer compositions, the reaction product of such pre-polymer compositions, and the diamine curing agent used in their reaction. While this material may offer improved lens characteristics over conventional materials, the inventors noted that it has too much residual yellowness for an acceptable standard ophthalmic lens. In addition, when the inventors worked with the disclosed prototype material to try to manufacture lenses, they noted that it does not have sufficient structural integrity to maintain an accurate optical power when surfaced with standard optical grinding, polishing, and edging techniques.
Due to the foregoing deficiencies, in order for the prototype material disclosed in the '617 patent to be an acceptable lens material, the inventors had the formulation modified (hereinafter “modified high impact polymeric material”). In particular, the inventors added dyes or colorants to obtain the specific requirements of a standard ophthalmic lens. The inventors also added stabilizers to protect the polyurethane component of the disclosed material from oxidation. Finally, the inventors modified the disclosed material's chemistry and component ratios to improve its structural integrity.
As shown in Table 1, the modified high impact polymeric material compares quite favorably with conventional optical lens materials in its combination of physical properties. Notably, the modified high impact polymeric material exhibits very low birefringence. This property is an especially useful attribute in combination with polarizers. Briefly, the polarizer in optical-quality eyewear has been aligned to preferentially remove most of the glare (plane polarized reflections) from horizontal surfaces. If a material has a high degree of birefringence (that is, if its crystal structure causes incoming light to be polarized significantly differently along different crystal planes), it will affect the apparent efficiency of a polarizing lens. If a birefringent material is now placed in the light path before the polarizer, some of this plane-polarized light will be redirected into other orientations such that the polarizer alignment will not block as much of the incoming light. The result is that the lens will be far superior to a tinted lens in blocking glare (since tinted lenses have no preferential absorption or reflection for plane polarized glare), but it will also not be as efficient as a lens without birefringent materials.
After modifying the prototype material disclosed in the '617 patent and analyzing its physical properties, the present inventors recognized that their modified high impact polymeric material could possibly be used in the manufacture of improved optical-quality plastic parts. The present inventors also recognized that if their modified high impact polymeric material could be combined with a polarizer, they might be able to provide the marketplace with improved polarized eyewear. Such optical-quality polarized parts include, but are not limited to, semi-finished, finished prescription and non-prescription lenses, facemasks, shields, goggles, visors, and display of window devices.
Initial tests, however, led the inventors to believe that their modified high impact polymeric material could not be utilized to manufacture optical-quality polarized plastic parts. In early attempts to combine their modified high impact polymeric material with standard polyvinyl alcohol (PVA) polarized film using conventional techniques, the film was consistently displaced and bent out of shape during the introduction of the material. Thus, initial testing revealed that a substitution of their high impact material for standard lens thermoset resin materials and conventional manufacturing processes was not possible.
Analysis of the initial testing further revealed that the properties of their modified high impact polymeric material greatly contributed to the inventors' failure to incorporate it into an improved optical-quality, polarized plastic part. Briefly, casting of polarized lenses and other eyewear requires controlled and reproducible positioning of the film or supported polarizer within the solidifying polymer. Gasket designs and certain conventional filling techniques typically help to control the positioning of the film during standard lens casting. It is not uncommon to spend 10 to 15 seconds filling the assembly with resin to ensure even flow and controlled distribution of the resin around the polarizer layer. However, their modified high impact polymeric material solidifies more quickly than standard thermoset resins (approximately 30 seconds rather than several hours). Thus, standard PVA polarized film was consistently displaced and bent out of shape during the introduction of the material due, at least in part, to the quick setting time of the material.
In a similar manner, the polarization or other essential physical properties of standard polarizing film can be compromised by the heat of the polymer's solidification process or by reaction with the monomers of the pre-mix. The modified high impact polymeric material creates considerable heat within the mold assembly during its normal, exothermic curing process. This can soften the polarizer or supporting layers, causing further displacement of the polarizing film. Depending on the polarizers or polarizing materials used, this heat could also change the color or decrease the efficiency of a polarizer. Organic dyes used as polarizers would be especially susceptible to this type of damage.
Thus, the inventors recognized that existing manufacturing processes suggested that high impact polyurethane-based material could not be used to effect an optical-quality plastic part due to the fundamental difficulty of handling the fast-reacting modified high impact polymeric material, in combination with the more demanding process of reproducibly positioning a polarizer within any optical construct, while maintaining the optical and mechanical performance of the part. If high impact polyurethane material could be incorporated into an optical-quality plastic part, a desirable product would be effected.